The present invention relates to an improved strapping machine and, in particular, a machine of the type for applying and tensioning a plastic strap around an object and forming a heat-sealed joint between overlapping portions of the strap.
This invention is an improvement of the strapping machine manufactured by Interlake, the assignee of the present invention, and described in an Interlake instruction manual entitled "G18 Strapping Head" (Form 1256), Copyright 1976, and partially disclosed in U.S. Pat. No. 4,058,053, issued to F. J. Patterson et al. on Nov. 15, 1977, and assigned to the assignee of the present invention.
In the aforementioned and other prior art strapping machines the heat-sealed joint in the plastic strap is typically formed by pressing overlapping strap portions against a heating element interposed therebetween for melting the facing strap surfaces, releasing the pressure to allow the straps to separate from the heating element, removing the heating element and then again pressing the melted strap surfaces together to form the joint, Such machines have performed quite well with plastic straps such as polypropylene, polyethylene and the like. But when polyester strap is used, these prior art devices have performed unsatisfactorily in bonding the overlapping strap portions together. More specifically, it has been found that with polyester strap, during heater retraction the melted strap surfaces start to resolidify before they can be joined together, thereby impairing or destroying the integrity of the bond.
It has also been found that, occasionally, the overlapping strap portions and/or the heating element will not be aligned exactly parallel to each other or exactly perpendicular to the line of travel of the movable pressing member. This condition causes uneven contact of the strap with the heating element and can interfere with retraction of the heating element, resulting in a nonuniform and imperfect bond and, heretofore, has been eliminated only by the provision of expensive, critically machined parts.
In U.S. Pat. No. 3,269,300, issued on Aug. 30, 1966, and U.S. Pat. No. 3,759,169, issued on Sept. 18, 1973, there are disclosed strapping machines which maintain pressure on the overlapping strap portions while the heating element is being withdrawn from therebetween, but these systems still suffer from the effects of nonparallel strap and/or heating element conditions, referred to above. The apparatus of the U.S. Pat. No. 3,759,169 applies pressure by moving a presser plate against the overlapping strap portions by means of a cam, a higher pressure being applied after the heating element has been withdrawn. But this system provides no means for effectively controlling the magnitude of the force applied during the melting and sealing phases of the pressing operation.
Also, in prior art machines it has been found that after repeated cycles of operation, residue from the melted strap portions tends to build up on the heating element. This residue serves as an insulator impairing the heating efficiency of the heating element and the residue can be deposited in future melted strap portions.
Furthermore, while the above-referenced prior art machines utilize a sensing device for sensing the completion of the loop for de-energizing the feed means, the Interlake machine has a tendency to coast, thereby overfeeding the strap past the sealing region, resulting in wasted strap. The machine of the U.S. Pat. No. 3,759,169 provides a positive stop for the strap, but the feed motor is de-energized by the actuation of a switch in response to the creation of a deliberate overfeed condition, subsequent to the stopping of the leading end of the strap. When the coasting tendency of the air motor after de-energization is added to this deliberate overfeed, there results a considerable accumulation of excess strap which must be withdrawn at the beginning of a tensioning operation, thereby significantly lengthening the time required for the tensioning operation. Also, this arrangement suffers from a tendency of the overfed strap to jam in the switch mechanism or other portions of the track.
Copies of the above-identified instruction manual and each of the aforementioned prior art patents are filed with this application, and they represent the most pertinent prior art of which applicants are aware.